Fluorescent lamp protection apparatus

ABSTRACT

A fluorescent lamp jacket assembly is disclosed employing an expansible or dilatable plastic jacket which substantially encloses the entire length of the glass envelope of a fluorescent lamp and is held to the lamp by means of a pair of plastic end caps so that rupturing of the glass lamp envelope will not be accompanied by bursting or separation of the jacket or its end caps, thereby containing all the debris ensuing from the implosion reaction accompanying breaking of the fluorescent lamp.

United States Patent Du Pont 1 June 27, 1972 [54} FLUORESCENT LAMPPROTECTION 3,136,489 6/1964 Oharenko ..240/! 1.4 APPARATUS 3,179,7924/1965 Weiss .240/ 1 1.4 3,358,167 12/1967 Shanks ..313/25 [72]Inventor: Paul R. Du Pont, Gillette, NJ.

731 Assignee: Thermoplastic Processes Inc., Stirling, NJ. P P QueisserAssistant Exammer-Elhs J. Koch [22] Filed: Oct. 29, 1969 Attorney-H0wardR. Popper [21] App]. No.: 872,321 ABSTRACT A fluorescent lamp jacketassembly is disclosed employing an 'i 340/112 expansible or dilatableplastic jacket which substantially en. 58] Fieid y 2 P 51 11 closes theentire length of the glass envelope of a fluorescent 240/1 3 i lamp andis held to the lamp by means ofa pair ofplastic end caps so thatrupturing of the glass lamp envelope will not be [56] References Citedaccompanied by bursting or separation of the jacket or its end caps,thereby containing all the debris ensuing from the implosion reactionaccompanying breaking of the fluorescent lamp.

4 Claims, 6 Drawing Figures PATENTEDJum I972 3, 678 401 sum 1 or 2 FIG.

INVENTOR R R. DUPONT 8V A 7'7'ORNEV PATENTEUJUNZ? I972 SHEET 20F 2 FIG.4

FIG. 6

FLUORESCENT LAMP PROTECTION APPARATUS BACKGROUND OF THE INVENTION Thisinvention relates to a shielding arrangement for a fluorescent lamp, andmore particularly, to a structure for reducing the explosion, shrapnel,and contamination hazard that may exist where such lamps must be used inlocations exposing the lamps to vibration, severe variations intemperature, splashing liquids and the like.

It has been appreciated for some time that the performance offluorescent lamps is enhanced when the lamp is insulated from lowtemperatures and severe winds. In an article in the Feb. l96l issue ofIlluminating Engineering, for example, the authors reported thatjacketing the fluorescent lamp allowed the lamp to produce a lightoutput over a temperature range that compared favorably with thatobtained from lamps enclosed in weatherproof luminaires. For fluorescentlamps, generally, it is known that light output, as affected by internalmercury vapor pressure, is related to the temperature of the coldestspot of the glass bulb envelope, maximum light output being obtained ata cold-spot temperature of approximately 100 F.

The above-referenced article described a glass-jacketed fluorescent lampwhich has since become quite popular despite the fact that its cost isappreciably more than that of the standard, unshielded lamp. The glassjacket of this prior art arrangement had a diameter approximately halfan inch greater than the nominal 1% inch diameter of the standardfluorescent lamp and its length was almost equal to that of the exposedportion of the lamp envelope lying between the terminal ferrules. Theglass jacket was fastened to the lamp by a pair of rubber bushings eachof which grasped not only the terminal ferrule but also the adjacentsurfaces of the lamp en velope. A somewhat earlier version of a jacketedfluorescent lamp is shown in L. R. Keiffer US. Pat. No. 2,363,109 issuedNov. 21, 1944 and a somewhat later version in H. Weiss US. Pat. No.3,179,792 issued Apr. 20, I965.

While the jacketed fluorescent lamp is thus known to stabilize lightoutput over a greater temperature range than is possible with a barelamp, present day jacketed arrangements are expensive and do not fullyprotect against the hazards of cuts from broken tubes. Such cuts areconsidered potentially serious not only because the glass may penetratedeeply because of the high velocity imparted to the shards by theimplosion reaction, but also because the phosphors and oxide powderscontained in the lamp may lead to infection and slow healing. Since mostfluorescent lamps are used in overhead fixtures, a lamp which falls isalmost certain to do so from a height sufficient to insure its breaking.

Of course, plastic-jacketed lamps have been introduced which are farless susceptible to shattering than glass jacketed lamps. However, eventhe plastic-jacketed lamps have thus far permitted the plastic jacket toseparate from the fluorescent lamp on impact and have therefore not beenreliable in containing the shrapnel if the impact was severe enough tobreak the lamp envelope within. While the jacket could be cemented tothe end bushings, it is desirable to reduce the cost of employingjacketed fluorescent lamps and, obviously, cementing the jacketprecludes the re-use of the jacket on a new lamp thereby preventing thecost of the jacket from being amortised over the life of several lamps.

Accordingly, it is an object of the present invention to pro vide animproved jacketed fluorescent lamp which is reusable and economical.

It is another object of the present invention to provide a fluorescentlamp jacket which will remain intact and contain the debris even afteran impact strong enough completely to shatter the lamp envelope within.

SUMMARY OF THE INVENTION The foregoing and other objects are achievedaccording to the principles of the present invention, in oneillustrative embodiment thereof, by providing a jacket assemblycomprising an expansible wall cylinder of polycarbonate or similarnonbrittle plastic having good heat resistance which is concentricallymounted over the lamp envelope and firmly held at each of its ends by anend cap'of high heat stabilized polypropylene which covers the terminalends of the lamp. Advantageously, the end caps grip only the terminalferrule portions of the fluorescent lamp rather than the lamp envelopeitself. The plastic jacket is made of a material having a stiffnessmodulus and wall thickness such that the sudden change in pressure,which may be occasioned by an impact of sufficient force to fracture theglass fluorescent tube envelope within the jacket,

will be dissipated by causing flexure in the jacket walls,

thereby preventing the force of the implosion reaction from beingdirected to the end caps. l have found polycarbonate tubing, having anominal wall thickness of 0.020 inches and an impact strength of 14foot-pounds on the Izod scale, to be satisfactory for this purpose, inone illustrative embodiment.

The ends of the plastic cylinder or jacket of my invention are held in aconically-tapered recess in each end cap. The recess holds the jacket sothat a twisting motion is required to remove the cap from the jacket andso that the longitudinal or radial forces that may be exerted in animplosion reaction or on impact with an object in a fall will be whollyincapable of causing separation of the cap from the jacket. Eachconicallytapered recess is further provided with a subgated vent throughwhich air that would otherwise be entrapped during assembly of thejacket and end cap may escape, thereby permitting a tighter assembly ofcap and jacket. Conversely, the vent acts as a vacuum break or air entrypoint to facilitate disassembly when it is desired to re-use the jacketwith another fluorescent lamp. The subgated vent is in each caseoperated by the torsional flexure imparted manually in assembling ordisassembling the jacket and end caps but is not operated by the radialor longitudinal forces that may be encountered on impact. Thepolypropylene end cap is closed by a webbed membrane which protects theend of the fluorescent lamp and, in the event that the lamp envelope isbroken by a severe impact, provides a vacuum seal so that the jacket andits end caps will remain together, thus containing the debris of theimplosion.

BRIEF DESCRIPTION OF DRAWING The foregoing and other objects andfeatures may become more apparent by referring now to the drawing inwhich:

FIG. 1 shows a portion of the jacket and one version of the end cap ofthe present invention assembled on a standard fluorescent lamp;

FIGS. 2 and 3, respectively, show an end view and a sectional viewthrough the rim and vent of the cap of FIG. 1;

FIGS. 4 and 5, respectively, show end and sectional views through analternative version of the end cap of the present invention; and

FIG. 6 shows a stop-action view of the end cap and jacket assembly ofthe present invention at the instant of implosion of the fluorescentlamp within the assembly.

GENERAL DESCRIPTION Referring now to FIG. 1, there is shown one-half ofa fluorescent lamp and one-half of one version of a jacket assemblyaccording to the present invention, it being understood that the otherhalf of the illustrative lamp and jacket assembly, which are not shown,are entirely symmetrical. The conventional fluorescent lamp comprises aglass envelope 9 of transparent or translucent glass containing theusual phosphors and oxides and enclosing a thermionic filament (notshown). Each end of the glass envelope 9 is provided with a terminalferrule 5 which provides for supporting an end plate (not shown), whichplate carries the electrical terminals 4. The construction of afluorescent lamp being well known, it is not necessary to describe thedetails thereof herein. It will, of course, be appreciated thatfluorescent lamps possess different types of end terminals, the mostcommon, two-pin type terminal 4, being shown by way of example inFIG. 1. Gripping the terminal ferrule about its circumference andsealing the end of the fluorescent lamp, in the region of its terminals4, is the end cap 2, which forms part of the assembly of the presentinvention. Inserted into and slightly distorting the tapered annularrecess 6 provided in end cap 2, is the expansible plastic shield 10, thedetails of the end cap construction being more particularly revealed inFIGS. 2 and 3.

Referring now to FIG. 3, there is shown a section of the end cap 2comprising part of the assembly of the invention, which section is takenalong the lines 3-3 depicted in FIG. 2. The end cap is closed at one endby webbed portion or membrane 3 which is pierced by apertures 3a and 3bto accommodate the electrical terminals 4 (FIG. 1) of the conventionalfluorescent lamp.

The inside cylindrical portion 2a of the cap 2 nearest the membrane 3has a diameter sufficient to insure a snug fit about the metal lampferrule 5. To the left of portion 2a, the inside of end cap 2 isenlarged to define a slightly tapered or bell-mouthed surface 2b whichdimensioned to be large enough not to bind on the glass envelope 9 ofthe fluorescent lamp. Surface 2b defines the interior portion of anextended, tapered shell section of the main body of cap 2, the exteriorportion 20 of which forms one wall for the recess 6.

Radially external to the cylindrical portion 2a, cap 2 is provided withan annular ring 7 which supports the outer lip section 8. The interiorsurface 8a of outer lip 8 and the exterior surface 2c of the extended,tapered shell section 2b-2c define a tapered annular recess 6 (FIG. 3)into which the tubular shield 10 fits. The facing tapered surfaces 20and 8a which define recess 6 are each free to flex during theinstallation of jacket 10 and to remain somewhat deformed by thepresence of the plastic jacket while retained in recess 6. The taperedsurfaces 2c and 8a provide for tightly engaging the plastic shield 10despite variations in its wall thickness or of the end cap 2 which arelikely to be encountered in production.

To facilitate assembly and disassembly of the end cap 2 with the jacket10, the end cap is provided with at least one vent 12, which is cut intothe annular recess 6 at one portion thereof in such a manner that therecess 6 is both deepened or subgated as well as widened by the vent.The vent 12 thereby permits air that would be entrapped in recess 6during the assembly of jacket 10 to the end cap 2, to escape, part ofthe entrapped air being expelled to the atmosphere and part beingadmitted to the region between the jacket 10 and the lamp envelope 9. Inpreventing the entrapment of air which would otherwise tend to limit theaxial engagement of end cap 2 with cylinder 10, the inner surface 3c ofthe webbed portion 3 of the end cap is brought into intimate contactwith the flat surface a of ferrule 5.

In one important aspect of the present invention, the materials out ofwhich the dilatable, cylindrical jacket and the end cap 2 are madeprevent the end cap 2 from becoming disengaged from jacket 10, even whenthe jacketed and end capped fluorescent lamp is deliberately struck ordropped with sufficient force to break the glass tube envelope 9. Inaccordance with this aspect of the invention, in one illustrativeembodiment thereof designed to protect a standard 40 watt fluorescentlamp, the end cap 2 was made of high heat stabilized polypropylene, theweb member 3 had a thickness of 0.018 inch and the annular recess 6 hadan axial depth of 0.4375 inch. While a somewhat different thicknesswebmay' be employed it may not be made so thick as to interfere with theinsertion of the lamp into the conventional socket or fixture nor sothin as to constitute a mere mold flashing. The subgated vent in therecess portion of the end cap had an axial depth of 0.4531 inch so thateven with the cylindrical jacket 10 bottomed in the recess 6, a gap 10aexists through which air may escape. v 1

The cylindrical jacket 10 is advantageously made of polycarbonatetransparent tubing having a nominal wall thickness of 0.020 inch and animpact strength of 14 footpounds on the Izod scale. In the approximate4-foot length of the 40 watt bulb, a jacket of this material hassufficient flexibility so that even if glass envelope 9 is broken,thereby causing a violent change in the internal pressure within theassembled jacket and end cap, the end cap will not come loose from thejacket. Maintaining the integrity of the assembly prevents the shards ofglass from the broken tube 9 from injuring personnel and also protectssuch personnel from the danger of infectious cuts.

FIGS. 4and 5 show end and sectional views of an alternative end capwhich is suitable for protecting a conventional single-terminalfluorescent lamp in accordance with my invention. FIG. 6 shows this endcap assembled together with a plastic shield 10' and a single-terminallamp. For convenience, reference numerals in FIGS. 4-6 which are similarexcept for the inclusion of the prime symbol to those employed in FIGS.1-3 are intended to denote corresponding features. The shield-receivingrecess 6' of the single-aperture end cap differs from that of thedouble-aperture end cap in that it is provided with a wide-mouth shallowchannel 15 to facilitate alignment of and assembly to cylindrical shield10'. In addition, the recess 6' has a somewhat less pronounced taper (aslittle as 6 illustratively) than the recess 6 of the embodiment shown inFIGS. 1-3. Further, shell section 2b and outer lip section 8' aresomewhat heavier than the corresponding features in FIGS. 1-3. Alsodifferent in detail is the subgated vent 12' which is now symmetricallyfaired into recess 6' and does not cut so much into the thickness of theouter lip 8' as did the vent of the first depicted embodiment. It shouldbe understood however that the design of the single-aperture anddoubleaperture end caps may interchangeably be used it merely v beingnecessary to provide the correct diameter and number of aperturesappropriate to the type of fluorescent lamp desired to be protected. Aswas true of the version of FIGS. l-3, the aperture 3ab' for the terminalof the lamp should have a diameter which is the same or even slightlysmaller than the lamp terminal in order to maintain the integrity of theenclosure formed by the end caps and jacket after the lamp is broken.

In FIG. 6 there is depicted a stop-action" view of a jacketedfluorescent lamp embodying my invention at the instant that the lampenvelope has been broken through some mishap. The breaking of the glassenvelope of the lamp causes a sudden drop in pressure within the jacket10' as the air which is normally in the space between the lamp envelopeand the jacket 10' rushes to occupy the vacuum within the now destroyedglass envelope 9 of the broken fluorescent lamp. However, because thejacket 10 of my invention is made of polycarbonate tubing or otherflexible material having similar properties, it is able to dissipate thepressure change through itself collapsing and later re-expanding whenthe momentum of the air in-rush momentarily causes a temporaryoverpressure within the confines of the jacket and end cap. The pressuredrop will normally be great enough with most present-day lamps to causethe terminals, whether of the single or double pin type, to be suckedinto the interior of the jacket. As is wellknown the withdrawal of theterminal pins will cause the lamp to fall from its socket or fixture ifit has not already done so because of the mishap which occasioned thebreaking of the lamp envelope inasmuch as the conventional fluorescentlamp depends on its terminals not only as the means for supplyingelectrical power but as the mechanical support within the fixture aswell. In accordance with one aspect of my invention, even if thejacketed fluorescent lamp should fall from its socket the end cap whichI have described herein will not separate from the jacket thusefiectively containing the shards of glass, the fluorescent powders andoxides and preventing them from coming into contact with personnel.

While I have described two illustrative embodiments of my invention itwill be apparent to those skilled in the art that modifications to therecesses 6 and 6 may be made as to the shape and size of the subgatedvent so long as the dimensions of the recesses and vents are not changedto make the fit of the jacket too loose.

What is claimed is:

1. A reusable protective assembly for a glass envelope fluorescent lamphaving terminal-bearing end-portions, comprising a cylindricaltranslucent plastic jacket having resilient walls,

said jacket having an axial length slightly shorter than the envelope ofsaid lamp, and

a plastic end cap disposed at each end of said jacket, each said end caphaving:

an internal periphery portion for frictional engagement with theperiphery of a respective one of said end-portions of said lamp,

an annular recess for receiving and providing a substantially air-tightseal with a respective end of said cylindrical jacket,

and a thin webbed section opposite said annular recess for closing theend of said cylindrical jacket;

said resilient walls of said jacket having sufficient flexure during animplosion of said lamp envelope to absorb the accompanying pressurechange thereby preventing separation of said end caps from said jacket.

2. A reusable assembly according to claim 1 wherein said annular recessof each of said endcaps is defined by a pair of concentricallypositioned, tapered wall sections, each of said pair of sections beingfree over a substantial portion of their length axial to said jacket toflex radially toward and away from said glass envelope of said lamp.

3. A reusable assembly according to claim 1 wherein said annular recessis provided with a subgated vent along a predetermined portion of itsperiphery to prevent entrapment of air in said recess therebyfacilitating assembly of said jacket with said end cap.

4. A reusable assembly according to claim 1 wherein said jacket is madeof poly carbonate tubing having an impact strength of approximately 14foot-pounds, lzod and wherein said end caps are made of high heatstabilized polypropylene.

i l i I

2. A reusable assembly according to claim 1 wherein said annular recessof each of said endcaps is defined by a pair of concentricallypositioned, tapered wall sections, each of said pair of sections beingfree over a substantial portion of their length axial to said jacket toflex radially toward and away from said glass envelope of said lamp. 3.A reusable assembly according to claim 1 wherein said annular recess isprovided with a subgated vent along a predetermined portion of itsperiphery to prevent entrapment of air in said recess therebyfacilitating assembly of said jacket with said end cap.
 4. A reusableassembly according to claim 1 wherein said jacket is made of polycarbonate tubing having an impact strength of approximately 14foot-pounds, Izod and wherein said end caps are made of high heatstabilized polypropylene.